How Much Bacteria Does The Human Body Really Contain?

The idea that every person contains 10 times more bacteria than human body cells is something of a myth. Juan Gaertner/Shutterstock
Ben Taub 11 Jan 2016, 18:34

The human body may contain around 10 times fewer bacteria than previously thought, with the average person being made up of roughly equal numbers of body cells and microbes. This information goes against the long-standing assumption that each living person is composed of around 10 times more bacteria than human cells, exposing this as something of a myth.

While invisible to the naked eye, the human body is in fact populated by vast numbers of microscopic life forms, collectively known as the human microbiota. This includes fungi and other microorganisms such as archaea in addition to bacteria, although it is the latter that are particularly abundant – so much so that for many years it was not thought to be inaccurate to consider each one of us as more bacteria than human. However, a new paper published in the journal bioRxiv suggests that we may have been underestimating our own humanness for the past several decades.

The study aimed to revisit previous research conducted in the 1970s that found that the number of bacteria in the human body may have been as high as 1014, or 100 trillion. This was calculated by measuring the amount of bacteria in a single gram of human feces as 1011 (100 billion), and multiplying this by 1,000, on the basis that these microbes are evenly spread around the digestive, or alimentary tract, which has a volume of one liter.

In contrast, the average human body is thought to contain about 1013, or 10 trillion cells, the majority of which are red blood cells. Hence, the idea that each person is 10 times more bacteria than human was born.

However, the authors of the new study challenge these previous findings, claiming that microbes are not evenly spread through the alimentary tract, but are instead concentrated mostly in the colon. As such, they calculated that each defecation actually contains around a third of the human microbiota.

Based upon this information, the researchers estimate that a “reference man” – who is aged 20-30, 1.7 meters tall (5 feet 6 inches), and weighs 70 kilograms (155 pounds) – is likely to contain around 3.9 x 1013 microbes in his body. At the same time, they calculate that the same individual should have around 3.0 x 1013 body cells, producing a ratio of 1.3:1.

While the authors concede that this ratio is likely to vary greatly between individuals due to differences in body mass and gut biology, they believe their findings disprove the myth that humans contain 10 times more bacteria than body cells.

Furthermore, they claim that each time their reference man defecates, the balance of his anatomical composition may be tipped so that he actually has more body cells than bacteria, since he loses about a third of his bacteria. In other words, going to the bathroom actually makes us more human!

Finally, the researchers insist that this reduction in the estimated number of bacteria in the human body does not detract from the presumed importance of our microbiota, and that while their study provides interesting anatomical information, it is unlikely to have major biological implications.

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